This invention relates generally to optical temperature sensing techniques, and more specifically to such techniques applied to measure the internal temperature of objects heated by electromagnetic radiation and/or ultrasonic energy.
There are many applications where the internal temperature of a fluid bath or more solid object is to be measured in an electromagnetic radiation and/or ultrasonic energy field that is heating the object. One such application is in the emerging field of medical hyperthermia, where tumors and cancerous tissue within a human body are heated by an external source as part of a program of medical treatment. For such treatment, the tumor is raised to a pre-determined elevated temperature and maintained there for a pre-determined period of time by directing electromagnetic radiation, in either the radio frequency or microwave spectrum, and/or ultrasonic energy from outside the body into the tumor. Each type of heating energy has advantages over the other for certain specific applications. Ultrasonic waves can be better focused and, because they are less strongly absorbed by tissue, are preferred for heating deep within the body. However, ultrasonic energy has disadvantages that include non-transmission through air or gas in a body cavity and high absorption by bone, resulting in electromagnetic energy being preferred when such body regions are involved.
In order to assure that the material being heated by either type of energy is maintained at the desired temperature, a non-perturbing temperature sensor is implanted in the tissue. In the case of cancer treatment of a human patient, such a sensor may be surgically implanted in the region to be heated prior to beginning the heating. Very small thermocouples and thermistors are traditionally used with either ultrasonic energy or electromagnetic radiation heating. More recently, fiberoptic temperature sensors have been employed to measure the temperature of materials heated by electromagnetic radiation. A significant advantage of the newer optical sensors is that they do not contain electrically conducting materials, thereby eliminating noise pickup and artifactual heating of the sensor that can result from the use of thermocouples and thermistors, as the result of electrical currents induced by the electromagnetic energy field. Also, the fiberoptic probes do not alter the incident heating field. Likewise, they do not conduct heat as strongly as do metallic wires or needles. As a result, measurements are more accurate in an environment having strong thermal gradients. For these reasons, significant sources of error in temperature measurements in an electromagnetic environment are eliminated.
Since the problem of induced currents does not exist with ultrasonic heating, however, thermocouples have until recently remained the technique of choice. Recently, however, it has been recognized that thermocouples also have problems in ultrasonic heating fields. Because of the acoustic mismatch, the thermocouple, unless extremely small, is driven relative to the tissue by the acoustic waves producing frictional or viscous heating. When plastic insulation is added or plastic catheters are used with thermocouples, absorptive heating is also observed. Finally, the thermo conduction of the metal leads can also produce errors in strong thermal gradients as noted above. Thus a better solution than thermocouples is also needed for ultrasonic heating fields. The fiberoptic sensors commonly used with electromagnetic heating contain plastics and therefore exhibit significant levels of absorption of ultrasonic energy. This causes an undesired measurement artifact from direct heating of the temperature sensor. Such artifacts are shown quantitatively in a poster paper entitled "Ultrasound-Immune Fiberoptic Thermometry Probe" that was given by applicants herein and others at the Annual Meeting of the Radiation Research Society, May 5-9, 1985. A solution suggested by this paper and related U.S. Pat. No. 4,626,110--Wickersheim and Sun (1986) is to select the materials and dimensions of the probe to minimize acoustic absorption, thermal conductivity and mismatch of acoustic impedance with the surrounding tissue. In order to minimize acoustic absorption, the fiber in the probe is selected to be quartz or glass because of its hardness.
It is a primary object of the temperature invention to provide an optical fiber temperature measurement sensor for use in an ultrasonic energy field that is a further improvement over that suggested by this poster paper and patent.
It is also an object of the present invention to provide an optical fiber temperature sensing probe that may be used with a variety of specific types of heating energy fields without itself introducing excessive errors.